Frequency modulation system



Oct. 1952 E. K. STODOLA 2,613,34

FREQUENCY MODULATION SYSTEM Filed Oct. 6, 1944 2 SHEETS-SHEET l CARRIER CHANNEL 22 I0 I I4 I2 38 CARRIER 90 PHASE R-F POWER osc. SHIFTER AMPLIFIER 26 PM on FM RECEIVER 120 /I6 /I8 MOD. SIG. BALANCED R-F POWER 24 GENERATOR MODULATOR AMPLIFIER I 30 B- SIDE-BAND CHANNEL FIG. 1.

FIG. 2. LINE OF MAX. 1

PHASE MOD. /E\Y M CARRIER I I 3 OSC.

TO NON-DIRECTIONAL.

ANTENNA I20 LAG hm- PHASE SHIFTER LINE OF MIN./ PHASE MOD.

I20LEAD FIG, 4. PHASE I I '1 SHIFTER I 1 I w??- so Q g] MOD. SIGNAL G ENERAI'UFZ TO DIRECTIONAL. ANTENNA INVENTOR. EDWI N K. STOD OLA Q ,2 .4 is .e 1.0 ATTORNEY RELATIVE AMPLITUDE OF RESULTANT 0i PHASE DI$PLACEIMENT OF- RESULTANT Oct. 7, 1952 STODOLA 2,613,348

FREQUENCY MODULATION SYSTEM Filed Oct. 6, 1944 2 SHEETS-SHEET 2 IQI I02 10:5 90 PHASE BALANCED R-F POWER I SHIFTER MODULATOR AMPLIFIER I v F|G.5. RIOS v 2 (H3 CARRIER BALANCED R-F POWER f osc. MODULATOR AMPLIFIER I I ll Q V 5 (I06 IL PULSE GENERATOR PM on FM I I20 (H6 RECEIVER E v L/I45 PULSE GENERATOR Ires L SWEEP GENERATOR l I 206 l INVENTOR.

EDWIN K. STODOLA Z/A/W W ATTORNEY modulation.

. components in Fig.5..

Patented Oct. 7, 195 2 UNITED STATES PATENT OFFICE FREQUENCY MODULATION SYSTEM Edwin K. Stodola, Neptune, N. J.

Applicaticnflct'ober 6, 1944, Serial No. 557,539

.16 Claimsi (01. 343- 11) (Granted under the act of M'ai'ch 3, 1"883Las amended April 30, 1928; 370 0. G. 757') The inventiondescribed herein maybe manutactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

' This invention relates generally towave-length modulation, and more particularly todirectional systems which utilize wave-length modulated waves.

It is a principal object. of this invention to provide a directional system in which an indication of direction is obtained bymeans of a wavelength modulated wave having a degree of. modulation which is a function of direction.

It is a further object of the invention to provide improved methods .and means for obtaining wave-lengthmodulated waves having. a high degree of linearity over a. relatively wide range of Still another object of the invention is to provide an improved objectdetectionsystem utilizing wave-length. modulated waves.

. In: accordance with this invention,v there. are

. transmitted. a plurality oflike-frequency phasedisplaced waves, preferably having di'iferent directional characteristics- One of said; waves is preferably of. constant amplitude while. another' isiamplitude modulated. Preferably one wave. is

omnidirectional whilev another is. directional. At a given point inspace .said. signals will combine 3 to form a resultant signal whichis wave-length modulated to adegree depending on the displacement ofsaid point from. the direction of maxi- For a better understanding: of: theinvention,

I'together with other and further obj'ectsthereof, reference is had. to the" following description taken Min connection with "the" accompanying M drawings and its scop'eWill be pointed outin the accompanying claims. V J

In theaccompanyingdrawings': Fig. 1 is a block diagram of one emb'odment of the invention;

Fig; 2 shows theradi'ation pattern ofwaves generated'by' the system in Fig. 1;

' Fig.3 is a block diagram of. another-embodi- 'ment of' the invention:

4. is .agraphdlmstratingj the. characteristicsof the system in. Fig. 3;.

Fig. 5 isa block diagram o'fia pulse-echo object detection system. incorporating. the invention; and

Fig. 6 shows wave forms. produced. bycertain Referrin particularly to. Fig 1, there is shown a directional system. comprising a radio-frequency -('R. -F.) carrier oscillator. 1L0}. preferably signal-to-noise ratio.

2 stabilized in frequency by means of' a crystal. The output of said oscillator is fed to a pair of channels A and 3 having. R..F. power amplifiers l2 and 18,. respectively. Channel 13 includes a balanced amplitude modulator l6, excited by both the carrier signal from oscillator Illand the output of a modulation signal. generator 26 which may generate a sinusoidal wave in the audiofrequency region. Balanced modulator It" will normally suppress all or part of the carrier and generate side bands having an instantaneous amplitude which is proportional to theinsta'ntaneous amplitude of the modulating voltage.

The R.-F. potentials in the two channels are phase shifted 90 with respect to each other by means ofan adjustable phase shifter M in the carrier channel .A. The phase shifter maybe inserted in the side-band channel, or a portion of the phase shift can be provided inboth channels. The output of channel A is of constant amplitude and is radiated by an antenna 22 which may be omnidirectional, while the output of. the side-band channel B is radiated by: a directional antenna 24, which may be a dipole array orloop antenna or equivalents" tliereof', of any desired degree of directivi'ty. Antenna 2'2 need only be relatively more broadlydirefc'tional compared to 24, ,that is, the signal-tonoise ratio may be improved by imp'artin'ga degree" of directivityto 22 ifthe needs of'the system permit.

For reception of said signals, a conventional phase-modulation receiver 26, havingan antenna 28 and output indicator 30, may be used. The antenna 28 may be"'omnidirectional, but'a' directional antenna may be used to increase the The receiver may incorporate an amplitude limiter" to eliminate undesirable amplitude modulation of the received signal.

Reference is now madeto Fig. 2 foran explanation of the operation of the system above described. Assuming that transmission takes place from a point 0, the circle V represents the omnidirectional radiation patternfrom antenna '22, while the directional antenna 24 has a figurethe directionally transmitted output of the-sideband channel "variesfrom' a minimum; whichis zero when the carrier is entirely suppressed by of-e'ightpattern W. The omnidirectional wave is transmitted with. constant amplitude, while the balanced modulatorlfi, to a maximum durmg each modulation cycle. p A phase-modulationreceiver positioned along line Y'--Y will receive both waves, the omnidirectional wave havin'ga fixed" amplitude; and the 90. phase-displaced directional signal being amplitude modulated. The ratio of said f nedin the receiver input circuit to yield a resultant "pendent upon direction.

It'should be noted that the maximum amplitude of the directional wave in the direction of maximum radiation should not exceed about I extent of modulation does not exceed about (tan 30=0.577)

Twill not be linear, and will result in distortion of the wave shape in the output" of the receiver I and reduce the sharpness of direction determinatlon'. Suitable control means (not shown) {characteristics with systems of this type, it is phase-modulated signal which will be translated byjth'e receiver.

A receiver positioned along line Z-"-Z will re- 'ceive both signals, the ratio of the amplitude of the unmodulated signal with respect to the maximum amplitude of the modulatedsignal being proportional to OM/ON'. SinceON is smaller than ON, the resultant signal at the receiver will have a lower degree of phase modulation.

A receiver along line X--X will receive a sideband signal having zero or minimum amplitude,

is dependent upon direction. Hence, the degree [or percentage of modulation of the resultant phase-modulated wave at the receiver is de- 0.57'7' of the fixed amplitude wave, so that the Modulation in excess of 30 for independently adjusting the relative amplitudes of both of the transmitted waves should, ftherefore, be incorporated in the two wave channels.

However, to obtain maximum noise reduction desirable to obtain phase modulation greater than 30. Asystem for producing such greater phase modulation isillustrated in Fig. 3, which shows three wavejchannels 50, 60 and I0 coupled ip to the carrier oscillator 40. Channel includes an amplifier 54, while channelsr and 10 include modulators and "I4,'each of which may incorporateconventi'onal means (not shown) for initially adjusting the output .amplitudes of cur-- rents therein. Channels 60 and I0 include phase shifters 62 and I2 to cause currents therein to respectively lag' and lead the current in channel 50 by an angle or about 120. Because there is no fixed "plate voltage applied to'their tubesfi fi the outputsofmodulators 64 and I4 are nor-- ,mally zero. "Modulators 64 and arealternajtely made operative by applying the modulation signal to the plate circuits thereof in push-pull by means of a push-pull transformer 80, whereby they alternately modulate in amplitude the waves impressedv upon their inputs. The output of amplifier 54 is radiated by a broadly directional or omnidirectional antenna, while the outputs of modulators and I4 are coupled to a com- 6 monoutput circuit and radiated by a directional antenna. It will be seen that the system in Fig.

- 3 provides means for omnidirectionally transmit- ..ting one wave of fixed amplitude, andftransmitting one of two waves which are alternately am-t- 348 H I 4 lation can be obtained over a range which is greaterthan 30. For'a comparison of the modulation characteristics of the system in Figs. 1 and 3, reference is made to Fig. 4, wherein curves A and A show the phase and amplitude modulation characteristics respectively of the system in Fig. 3; while curves B and B show the phase and amplitude modulation characteristics respectively of the system in Fig. 1. Curve A indicates that the phase modulation of the system in Fig.3 is linear for as much as 60, while curve A indicates that the amplitude modulation of said system is considerably reduced, so that less amplitude limiting will be needed in the receiver. I i

The phase modulaiton method in Fig. 3 is not restricted to a directional system wherein the wave components are combined in the receiver. This method can be used for conventional signaling purposes by combining the waves in'the three channels in a common output circuit at the transmitter to provide a resultant phase or frequency modulated wave which can then be multiplied to increase the degree of modulation and then amplified in theusual manner. The amplified resultant wave can then be radiated from an antenna. j V J The systems above described are capable of a variety of uses. If the directional wave isfixed in direction, the system is useful for producing a guide path for mobile craft. "By providing at the receiver an indicator calibrated in terms of degree of modulation the invention-can beused as a radio range or beacon. I

The invention can also be used for, pulse-echo object detection by'means or a receiver located at the point of transmission and by rotation of the directional antenna until an echo of -maximum degree of modulation is received. Pulse modulation can be applied to either channel,- preferably the more directional channel, or to both channels. In the latter case the waves in the respective channels should preferably be differentially modulated in amplitude. V I

Fig. 5 illustrates an embodiment of such'an object-detection system. As in Fig. 1, two wave transmitting channels C and D are provided, both being fed by a single frequency-stabilized carrier oscillator I00. The waves in said channel are phase-displaced with respect to "each other by a phase shifter IOI. Channels C and D are respectively provided with balanced modulators I02 and H2, R.-F. power'amplifiersII03 and H3, and'directional antenna arrays I04 and H4, provided with reflector screens I05 and H5." jBalanced modulators I02 and H2 will normally suppress the carrier impressed thereon softhat normally there will be no energy'radiated-from either antenna. I

The waves in channels C and D are periodically modulated by pulses derived from pulse generators I06 and H6 applied tomodulators l02 a nd I I2 respectively. The pulse generators are locked in synchronism by a syncronizing oscillator I20 operating at a relatively low frequency generally in the audio region. For each cycle or plurality vof cycles of voltagefrom oscillator I20, generators I06 and H6 will produce, in a manner well known in the art, a single sharp pulse,the duration of which is considerably shorter than the intervals between pulses. I

' Oscillator I20 also controls a sawtooth sweep generator I25 whichgenerates one sawtooth wave for each pulse generated by pulse generators I06 and H6. The sawtooth waves are applied to the Fig. 6; while pulses 206 are rectangular and preferably pulses 2I6. I25 generates sawtooth waves 225 in synchronous relation with pulses ZIE. vals between pulses 2 l6 depend upon the maximum distances to be measured, while the maxi- .mum duration of. sawtooth waves 225 is prefer-- .horizontaluplates of; a cathode'zray oscilloscope 130 to" provide. a time base.

.The output of a conventional PM orFM. re-

ceiver, fed by a directional. receiving antenna including a dipole array I40 andrefiector screen 145, is applied. to the vertical plates of oscilloscope tube I30. I V

The operation of the system in Fig. .5'will now be described with reference to Fig. 6. As above pointed out, there are normally no waves transmitted by channels C and D. Pulse generators I06 and H6 will synchronously generate pulses of the form shown at 206 and 2 I6 respectively, in Pulses 2H5 are preferably triangular.

wider than pulses I60. Said pulses are so synchronized that pulses 20E completely overlap At the same time sweep generator The minimum interably not greater than saidintervals.

Pulses from generators I 06 and I I6 are applied to balanced modulators I02 and I I2, respectively,

to modulate waves in channels C and D to produce. substantially coincident wave-trains having envelopes similar to those shown at 206 and 2&6, said wave-trains being separately radiated by ,antennas I04 and I I4. On striking a reflecting object, both wave trains will be picked up by antenna I40 of the receiver and vectorially combined in the antenna circuit of receiver I35 to produce a resultant wave-train. Since the wavetrain component radiated from channel C is substantially constant in amplitude for the pulse duration, while wave-train component radiated "from channel D i's'continuously varied in amplitude for the duration of the pulse, and since said wave-train components are 90 phase-displaced with respect to each other-,- the resultant wavetrain at thereceiver input is modulated in phase and translated by the phase modulation detector provided in the receiver to produce a vertical deflection of the oscilloscope trace. Thus a series of discrete phase-modulated wave-trains are pro- 90" with respect to each other, and then vectori'ally combining them in a receiver to produce a resultant wave-train which is modulated in phase.

As in the previously described systems, antenna 'll4should bedirectional, the directivity being as sharp as is desiredor as conditions may re- 'quire, while antennas I04 and I40 may be nondirectional or directional, but to a lesser degree *than that of antenna H4.

All directional antennas should be mounted for rotation in unison.

Although a separate antenna is used for reception; antenna I 04 can be used for this purpose by-providing the usual duplexing means.

Instead of separately transmitting the two wave components from channels C and D, they may first be combined'and then transmitted from a common antenna.

Modulators I02 and H2 need not be of the balanced type.

They may be amplifiers which are normally biased to cutoff and intermittently rendered conducting by the pulses from gener- I at ors I06 and H6. Or power amplifiers I03 and 11 3: may be so operated,

"6 Although. the above: description. ideals with phase modulated waves, it is well known that phase. modulation of a wave is alwaysha'ccompanied by a frequency modulation. Whereuonly one modulating frequency is involved, the two types of modulation are identical. .Hence the term wave-length modulation, as. usedwherein, is

meant to be generic to both types ofmodlulation.

While therehave been described what 'areat present considered preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the inventiomand it'is aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

I claim:

1. A wave-length modulation system comprising a source of carrier waves, three channels fed by said source, means for phase-displacing the waves in said channels approximately; with respect to each other, means for maintaining the wave amplitude in one channel substantiallyconstant, a modulator in each of the other two channels, and means adapted to be coupled to a source of signal energy ,for alternately energizing, said modulators, and means for combining the outputs of said three channels to derive aresultant'wavelength modulated wave.

2. 1A wave-length modulation system comprising a source of carrier waves, three channels fed by said. source, means for phase-displacing the waves in said channels approximately 120 with respect to each other, means for maintaining the waveamplitude in onechannel substantially constant, a normally non-conducting modulator in each. of the other two channels, said modulators having a common output circuit, means adapted to be coupled to a source of signal energy for controlling said modulators in push-pull, and means for combining the outputs of said three channels to derive a resultant wave-length modulated wave.

3. A directional radio system comprising a source of carrier waves, three channels fed by said source, means for phase-displacing the waves in said channels approximately 120 with. respect to each other, means for maintaining the wave amplitude in one channel substantially constant,

an antenna for radiating said constantamplitude wave, a normally non-conducting modulator in each of the other two channels, said modulators having their output circuits coupled to a second antenna, and means adapted to be coupled to a source of signal energy for rendering saidmodulators alternately conducting, one of said am tennas being more directional than the other.

4. A directional radio system comprisingv a source of carrier waves, three channels fed in parallel by said source, means for phase-displacing the waves in said channels approximately 120 with respect to each other, means for maintaining the wave amplitude in. one channel substantially constant. a first antenna for radiating said constantamplitude wave, a normally nonconducting modulator in each of the other two channels, said modulators having their output circuits coupled in parallel to a second antenna, and means adapted to be coupled to a source of signal energy for controlling said modulators in push-pull to render them alternately conducting, said first antenna being relatively non-directional and said second antenna being directional.

5.. A system. as set forth. in" claimv 4,.including' .ra wave-iength modulation receiver having a relatively non-directional antenna connected thereto, and means in the output of said receiver to indicate the degree of wave-length modulation :of the received signal.

' 6. A pulse signaling system comprising a source of constant frequency carrier waves, a pair of channels fed in parallel by saidsource, a normally blocked modulator in each channel, separate wave radiators coupled to the outputs of said modulators,'one of said radiators being more directive than the other, means for phase displacing the radiated wavesless than, 180;withrespect to eachother, means for synchronously generating two substantially concurrent groups of discrete signal pulses; the pulses of one group group of pulses and the'output of the other modu-1 lator with the" other group of pulses, a receiver :forr'eceiving and combining both groups of received pulses to produce resultant wavelength modulated pulses, said receiver including means for detecting only the wavelength modulation;

offsald resultant pulses, and a visual indicator controlled by theoutput of said detecting means.

7. 'A pulse-echo object-detection system comprising a source of constant frequency carrier waves, a pair of channels fed in parallel by said' ,means for phase displacingthe wave radiated byone antenna substantially 90 with respect to the wave radiated by the other antenna, means for synchronously generating two substantially concurrent groups of discrete signal pulses one from substantially the same" point in said medium with different degrees of di'rectivity, meansy-in at least one of said channels for amplitude modulating the energy therein, and means in at least one of said channels for producing a substantially constant phasedisplacement of less than 180 between the radiated wave fields at all points in said medium about said point, whereby when the'radiated waves are received and combined at any point in said medium they produce a resultant wavelength modulated signal having a degree of modulation which is dependent'upon direction. 1 a

10. A system for radiating a: direction deit pendent wave pattern comprising a source of wave energy, a pair-of radiating channels coupled to said source for separately radiating said waveenergy from substantially'the samepoint in said medium with different degrees of directivity, means in both of said channels for suppressed-carrier modulating the energytherein, and means in at least one of said channels for producing a substantially 90 phase displacement between the radiated-wave fields at -all points in said medium about said point, whereby when the radiated waves are received and combined at any point in said medium they produce 01' whichis rectangular and the other triangular, $40

with one group of pulses and the other modulator with the other group of pulses, a receiver for receiving and vectorially combining both modulated resultant pulses, said receiver including a detector responsive only to the wavelength modulation of said resultant pulses, a cathode ray indicator having beam controlling means controlled by the output of said detector, and

' means for providing a time base for said cathode ray' tube including means for generating saw- 1 tooth waves in synchronism with said pulses.

8. A communication system comprising means i'or generating carrier wave oscillations, means for omnidirectionally radiating said oscillations with a constant amplitude to produce a first 'iirst-namediradiating means for radiating said last-named oscillations to produce a second electromagnetic wave field, means for phase displacing said first andsecond electromagnetic wave fields substantially .90" at substantially every point in space, means for receiving and combining said electromagnetic waves to derive a resultantwavelength-modulated signal, and means fQr'detecting onlythe wavelength modulation .of

said signal.

-9. A system for radiating adirection dependent 1 wave pattern comprising a source of wave energy,

a pair-"oi radiating channels coupled tosaid sourcefor separately radiating said wave energy means for controlling one of said modulators groups of received pulses to produce wavelength 51.45

a resultant wavelength modulated signal having a degree of modulation which is dependentupon direction. j a

11. A system for radiating a direction ,dependent wave pattern comprising a source of wave energy, a pair of radiating channels coupled to said source for separately radiatingsa'id wave energy from substantially the same point in' said medium with different degrees of directivity, means in one of said channels forsu ppressed-carrier modulating the energy therein, the ratio of the amplitude of the modulated radiated wave to the amplitude of "the other radiated wave being not greater than 0.577, and means in at least one of said channels for producing a substantially phase displacement between the radiated wave fields at all pointsfin said medium about said point, whereby when the radiated 'waves are received and combined at any point insaid medium they produce a resultant wavelength modulated signal havinga degree of modulation which 'is dependent upon direction. v I J '12. A directional signaling system comprising a source of wave energy, a pairoi. radiating channels coupled to said source for separately radiating said wave energy from substantially the'same point in said medium with difierent degrees of directivity, means in atleast one of said channels for amplitude modulating the energy therein, means in at least one of said channels for producing a substantially constant phase displacement'of less than between, the radiated wave fields at all points in said medium about said point, and a receiver .for receiving and combining the radiated waves tofproduce a resultant wavelength modulated signal havinga degree of modulation which is dependent upon direction,

said receiver including means for detecting "only.

the wavelength modulation component of said signal.

vl3. A system. for radiating a direction dependfigure-of-eight directional pattern from substantially the same point, means for amplitude modulating the waves in at least one of said channels, and means in at least one of said channels for producing phase displacement of substantially 90 between the radiated wave fields at all points in said medium about said point, whereby when said waves are received and combined at any point in said medium they produce a resultant phase modulated signal having a degree of phase modulation which is dependent upon direction.

14. A system for radiating a direction dependent wave pattern in a medium, comprising a source of continuous waves, a first radiating channel for non-directionally radiating said waves from a given point insaid medium with constant amplitude, a second radiating channel for radiating said waves with a figure-of-eight directional pattern from substantially the same point, means in said second channel for suppressed-carrier modulating the waves therein, and means in at least one of said channels for producing a phase displacement of substantially 90 between the radiated wave fields at all points in said medium about said point, whereby when said waves are received and combined at any point in said medium they produce a resultant phase modulated signal having a degree of phase modulation which is dependent upon direction.

15. A system for radiating a direction dependent wave pattern in a medium, comprising a source of continuous waves, a first radiating channel for non-directionally radiating one of said waves from a given point in said medium, a second radiating channel for radiating the other of said waves with a figure-of-eight directional pattern from substantially the same point, means in both of said channels for amplitude modulating the waves therein, and means in at least one of said channels for producing a substantially constant phase displacement of substantially 90 between the radiated wave fields at all points in said medium about said point, whereby when said waves are received and combined at any point in said medium they produce a resultant phase modulated signal having a degree of phase modulation which is dependent upon direction.

16. A system for radiating a directionally characterized signal comprising means for generating separated carrier wave and side-band components representative of a modulated carrier wave, a pair of radiating means for respectively radiating said components separately from efiectively the same point in space, one of said radiating means being more directive than the other, and means for producing a substantially constant phase displacement of substantially 90 between the radiated wave fields produced by said components at all points in space about said point,

whereby when said fields are received and combined at any point in space they produce a resultant space modulated signal having a degree of phase modulation which is dependent upon direction.

EDWIN K. STODOLA.

REFERENCES CITED The following references are of record in the file of this patent:

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